690 



NOTES TO BOOK XVIII. 



ledge, but of errour and confusion ; for in such applications 

 the real questions are slurred over in the hypothetical 

 assumptions of the mathematician, while the calculation 

 misleads its followers by a false aspect of demonstration. 

 All symbolical reasonings concerning the fissures of a semi- 

 rigid mass produced by elevatory or other forces, appear 

 to me to have turned out valueless. At the same time it 

 cannot be too strongly borne in mind, that mathematical 

 and mechanical habits of thought are requisite to all clear 

 thinking on such subjects. 



(HA.) p. 615. A point of Geological Dynamics of 

 great importance is, the change which rocks undergo in 

 structure after they are deposited, either by the action of 

 subterraneous heat, or by the influence of crystalline or 

 other corpuscular forces. By such agencies, sedimentary 

 rocks may be converted into crystalline, the traces of organic 

 fossils may be obliterated, a slaty cleavage maybe produced, 

 and other like effects. The possibility of such changes was 

 urged by Dr. Hutton in his Theory ; and Sir James Hall's 

 very instructive and striking experiments were made for 

 the purpose of illustrating this theory. In these experi- 

 ments, powdered chalk was, by the application of heat 

 under pressure, converted into crystalline calcspar. After- 

 wards Dr. McCulloch's labours had an important influence 

 in satisfying geologists of the reality of corresponding 

 changes in nature. Dr. McCulloch, by his very lively 

 and copious descriptions of volcanic regions, by his repre- 

 sentations of them, by his classification of igneous rocks, 

 and his comprehensive views of the phenomena which they 

 exhibit, probably was the means of converting many geo- 

 logists from the Wernerian opinions. 



Rocks which have undergone changes since they wei 



